Laboratory pipettes may include one or more shafts that may be removable. The shafts generally include a tapered external shape that can be fitted with a removable, disposable tip. Conventionally, the tips are force fitted on the shaft and held in place by friction. A longitudinal section through the inner surface of the tip is generally conical with dimensions that provide a frictional contact with the shaft sufficient to support a suitable retaining force and an acceptable leak tightness. The tip generally is ejected using an appropriate device integrated into the pipette. An exemplary ejection mechanism is described in French Patent No. A-2 807 342.
Fully satisfactory usage properties, however, are not obtained using existing pipette shafts because the force necessary to force fit the tip on the pipette shaft is uncertain and can be very large because there is no means of limiting this force, because contact pressure at the sealing area may be low resulting in an uncertain and non-reproducible seal quality, and because the force to be applied to eject the tip is uncertain and can be very large if the tip was force fitted onto the shaft with a high force and insertion depth. Additionally, because the height and position of the sealing area are not sufficiently well controlled, there is no guarantee that the position of the tips is correct. This is particularly a problem using multi-channel pipettes that include several shafts in line with each other on which the corresponding number of tips is force fitted. There is no guarantee that the position of all the tips is correct, and therefore, that they all provide a satisfactory seal at the contact with the corresponding shaft.
PCT Publication No. WO 2003/002980 includes a proposal that the tip —shaft contact be sealed by providing an annular rim on the shaft. This rim provides a local thickening of the shaft at which a contact surface formed on the inner wall of the tip stops. However, this rim tends to make it more difficult to insert the tip and requires tips with an unusual conformation and deformability in the contact area between the tip and the rim. Therefore, what is need is a pipette shaft that provides an improved solution to the problems discussed above.